A method for the in vitro evolution of Nucleic acid molecules with highly specific binding to target molecules has been developed. This method, Systematic Evolution of Ligands by Exponential Enrichment, termed the SELEX process, is described in U.S. patent application Ser. No. 07/536,428, entitled "Systematic Evolution of Ligands by Exponential Enrichment", now abandoned, U.S. patent application Ser. No. 07/714,131, filed Jun. 10, 1991, entitled "Nucleic Acid Ligands", now U.S. Pat. No. 5,475,096, U.S. patent application Ser. No. 07/931,473, filed Aug. 17, 1992, entitled "Methods for Identifying Nucleic Acid Ligands", now U.S. Pat. No. 5,270,163 (see also WO91/19813), each of which is herein specifically incorporated by reference. Each of these applications, collectively referred to herein as the SELEX Patent Applications, describes a fundamentally novel method for making a Nucleic acid ligand to any desired target molecule.
The SELEX method involves selection from a mixture of candidate oligonucleotides and stepwise iterations of binding, partitioning and amplification, using the same general selection scheme, to achieve virtually any desired criterion of binding affinity and selectivity. Starting from a mixture of Nucleic acids, preferably comprising a segment of randomized sequence, the SELEX method includes steps of contacting the mixture with the target under conditions favorable for binding, partitioning unbound Nucleic acids from those Nucleic acids which have bound specifically to target molecules, dissociating the Nucleic acid-target complexes, amplifying the Nucleic acids dissociated from the Nucleic acid-target complexes to yield a ligand-enriched mixture of Nucleic acids, then reiterating the steps of binding, partitioning, dissociating and amplifying through as many cycles as desired to yield highly specific, high affinity Nucleic acid ligands to the target molecule.
The SELEX method encompasses the identification of high-affinity Nucleic acid ligands containing modified nucleotides conferring improved characteristics on the ligand, such as improved in vivo stability or improved delivery characteristics. Examples of such modifications include chemical substitutions at the ribose and/or phosphate and/or base positions. SELEX-identified Nucleic acid ligands containing modified nucleotides are described in U.S. patent application Ser. No. 08/117,991, filed Sep. 8, 1993, entitled "High Affinity Nucleic Acid Ligands Containing Modified Nucleotides", that describes oligonucleotide containing nucleotide derivatives chemically modified at the 5- and 2'-positions of pyrimidines. U.S. patent application Ser. No. 08/134,028, supra, describes highly specific Nucleic acid ligands containing one or more nucleotides modified with 2'-amino (2'-NH.sub.2), 2'-fluoro (2'-F), and/or 2'-O-methyl (2'-OMe). U.S. patent application Ser. No. 08/264,029, filed Jun. 22, 1994, entitled "Novel Method of Preparation of Known and Novel 2' Modified Nucleosides by Intramolecular Nucleophilic Displacement", describes oligonucleotide containing various 2'-modified pyrimidines.
Given the remarkable ability of Nucleic acid ligands to be generated against many different target molecules, it would be desirable to have methods for using said ligands as a diagnostic tool. In particular, it would be desirable to attach a plurality of different Nucleic acid ligands to a microfabricated solid support (a "Biochip"), and then assay the binding to said ligands of target molecules in a Bodily fluid. The subject application provides such methods.